This invention relates generally to compositions and methods for the formation of protective, corrosion-inhibiting coatings on metals, or other materials coated with metals, without the use of chromium in the hexavalent oxidation state. More particularly, this invention relates to non-toxic, corrosion-inhibiting conversion coatings based on tetravalent cerium, praseodymium, or terbium and methods of making and using the same.
Metals like aluminum, zinc, magnesium, titanium, cadmium, silver, copper, tin, lead, iron, rare earths, zirconium, beryllium, niobium, tantalum, lithium, or indium, their alloys, or items coated with these metals, tend to corrode rapidly in the presence of water due to their low oxidation-reduction (redox) potentials or ease of oxide formation. Non-alloyed specimens of these metals typically form a natural oxide film that will protect them somewhat and reduce their overall rate of corrosion. However, alloys of these metals are particularly sensitive to corrosive attack. These materials also have a significant problem with paint adhesion. The as-formed metal surfaces are typically very smooth, and they tend to form weakly bound surface oxides. The native oxides do not normally provide a robust base on which subsequent paints can anchor themselves. These metal alloys have many uses ranging from architectural adornments, to protective coatings on ferrous alloys, to structural aerospace components, inter alia.
The 2000 and 7000 series of aluminum alloys are used throughout military and civilian aircraft because of their high strength to weight ratio. However, these aluminum alloys are very sensitive to corrosive attack because their natural oxide layer offers only a limited degree of protection. Materials with greater redox potentials, such as steels or carbon fibers, in proximity to aluminum alloys will promote corrosive attack in water by the formation of a galvanic corrosion couple with the less-noble light metal alloy.
Inhibiting the initiation, growth, and extent of corrosion is a significant part of component and systems design for the successful long-term use of metal objects. Uniform physical performance and safety margins of a part, a component, or an entire system can be compromised by corrosion.
One method of enhancing the corrosion resistance of these alloys includes the use of a conversion coating. A conversion coating is a self-healing, corrosion-inhibiting layer formed during intentional exposure to a chemically reactive solution. The conversion coating process forms an adherent surface containing an integral corrosion inhibitor with “throwing power” that can provide protection to coating breaches. The metal is exposed to a compound that chemically alters the surface and forms a coating that provides a high degree of corrosion resistance. A chemical conversion coating applied to the surface of a less-noble alloy can reduce the extent and severity of aqueous corrosion, provide long-term property stability, and extend the useful life of the object of manufacture.
Conversion coatings incorporate a portion of the base metal and form a mechanical, chemical, and electrostatic barrier to corrosive attack. A feature of effective conversion coatings is their ability to provide corrosion protection to the base metal in the presence of a coating breach.
Anodization of a metal surface followed by “sealing” or “rinsing” of the anodized metal does not constitute the formation of a conversion coating in our usage. Anodization, the formation of a porous oxide film on the metal, is achieved by the application of an electrical potential to the metal. This oxide film must then be “sealed”, “washed”, or “rinsed” in order to impart complete corrosion protection. Typically, the corrosion protection afforded by an anodized piece is due to the barrier oxide film. Conversion coatings, however, grow an oxide coating on the metal without an externally applied electrical potential. The protective film is produced by a chemical redox reaction between the metal surface and the conversion coating solution. The film is composed both of an oxide and integral corrosion inhibitor species formed during exposure to the conversion coating solution. A true conversion coating therefore affords corrosion protection from an oxide barrier film that has co-deposited oxidative corrosion inhibitor species.
A conversion-coated surface may be left bare or afforded further protection by the application of additional films or coatings. Conversion coatings need to adhere to the substrate and should result in a surface that will promote the formation of a strong bond with subsequently applied coatings. Bonding with subsequently applied coatings is a function of the morphology and chemical composition of the conversion coating. Adhesion promoting surface treatments may exhibit corrosion-inhibiting characteristics. Depending on the intended application, a conversion coating, as described herein, may be considered to be an “adhesion promoter” and vice versa.
Conversion coatings are usually formed by the application of a conversion coating solution to a metal surface. The solution can be applied by immersion, spray, fogging, wiping, or other means.
Hexavalent chromium has traditionally been used in the formation of protective conversion coatings for aluminum, zinc, magnesium, titanium, cadmium, silver, copper, tin, lead, iron, rare earths, zirconium, beryllium, indium, and their alloys. Compounds such as Alodine® (available from Henkel Surface Technologies, Madison Heights, Mich.) and Alumigold™ (available from Turco Products, Inc., Madison Heights, Mich.) contain hexavalent chromium as their main corrosion-inhibiting compound.
Two generic types of hexavalent chromium coatings have been widely used. The newer “gold” coatings are named for the faint gold tint that is exhibited when these coatings form on the surface of aluminum alloys. The compositions and application procedures of these “gold” hexavalent chromium conversion coating formulations are described in United States military process specifications, as well as other federal guidelines. Therefore, guidelines for the application of these solutions to aluminum (MIL C-5541; MIL C-81706; MIL STD-171; ASTM B-449), zinc (ASTM B-633; ASTM B-201; MIL C-17711; QQ Z-325a), magnesium (MIL M-3171), cadmium (ASTM A-165; ASTM B-201; QQ P-416b), silver (ASTM B-700; QQ S-365a), copper (ASTM B-281), and tin (ASTM A-599; QQ-T-425a) are available. The common components to these “gold” conversion coating baths are hexavalent chromium, complex fluorides, and ferricyanide. Older “green” conversion coatings containing hexavalent chromium have also been described, and the color formed on aluminum alloys through the application of these conversion coatings is a light green color. The “green” formulations all contain hexavalent chromium, a fluoride, and an acidic phosphate component. The major compositional difference between the two is that the current “gold” formulation contains ferricyanide and the older “green” formulations contain phosphate.
Corrosion-resistant compositions have also been described which contain hexavalent chromium, fluoride, and molybdic acid or molybdates, rather than ferricyanide or phosphate. Tungstates and vanadates have also been used in combination with hexavalent chromium and fluoride. Hexavalent chromium formulations which do not contain a fluoride source, and which contain borate ions instead of ferricyanide or phosphate or molybdate have also been described. Hexavalent chromium has also been used in combination with stannates, oxalates, and tellurates. Finally, corrosion protection of aluminum, magnesium, or zinc alloys has been achieved through the use of hexavalent chromium, fluoride, and rare earth compounds.
The variation in the type and amount of additional components such as ferricyanide, phosphate, molybdate, and borate, etc., in conversion coat formulations based on hexavalent chromium is significant in light of the chemistry developed and presented in the present invention. It is important to note that hexavalent chromium conversion coatings which have nearly identical formulations, except for one or more of the non-chromium components, result in obvious differences on the applied metal surface for a given alloy (such as “gold” and “green” coatings). It is also important to note that differences in the composition of aluminum alloys will influence the chemistry of the conversion coating formed when only one hexavalent chromium conversion coat composition is used.
Significant efforts have been made to replace chromium with other metals for corrosion-inhibiting applications due to toxicity, environmental, and regulatory concerns. Cerium is one non-toxic, non-regulated metal which has been considered as a chromium replacement. Cerium (like chromium) exhibits more than one oxidation state (Ce+3 and Ce+4). In addition, the oxidation-reduction potential of the Ce+4—Ce+3 couple is comparable to the Cr+6—Cr+3 couple. For example, in acid solution:Ce+4+e−→Ce+3+1.72 VCr+6+3e−→Cr+3+1.36 V
Praseodymium and terbium also exhibit more than one oxidation state (Pr+3 and Pr+4, Tb+3 and Tb+4). Tetravalent praseodymium and terbium are even stronger oxidizing agents than cerium (with calculated redox potentials of +3.3 V in acidic solution (Nugent, L. J., et al., J Inorg. Nucl. Chem. 33: 2503-30, 1971):Pr+4+e−→Pr+3+3.2 VTb+4+e−→Tb+3+3.2 VCr+6+3e−→Cr+3+1.36 VA number of processes have been reported in the literature that make use of cerium in conversation coating bath solutions, as well as general corrosion protection or coloring of the alloys. However, the coatings formed by these processes provide only limited protection and do not approach the benefit derived from the use of hexavalent chromium.
The use of film-forming substances, such as polymers, silicates, sol-gel, etc., which have no inherent oxidizing character, in conversion coating solutions has been described in the literature. The film formers may enhance short-term corrosion resistance by functioning as a barrier layer. However, these films interfere with substrate oxidation during the conversion coating process and produce thin, incompletely anodized surfaces, resulting in poor mechanical adhesion to the solution-deposited polymer film and to later applied coatings. Restricting the formation of the oxide layer that acts as a reservoir for the active corrosion inhibitor yields a barrier film that is inhibitor starved. Barrier layers lacking an active corrosion inhibitor have been demonstrated to be capable of inhibiting corrosion only as long as the barrier is not breached, as by a scratch or other flaw. Film formers can actually enhance corrosion on a surface after failure due to the well-known effects of crevice corrosion. The addition of polymer during conversion coating also produces a smooth coating which can reduce subsequent paint adhesion, resulting in reduced long-term corrosion protection.
Likewise, a myriad of inorganic oxide, phosphate, silicate, carbonate, ox al ate, molybdate, tungstate, zirconate, titanate, borate, etc. barrier films have been described in the literature as providing corrosion protection. However, these films will serve this function so long as the film or coating is not breached to expose bare metal. Should this occur, none of these coatings exhibit “self-healing” characteristics. For example, hexavalent molybdenum (Mo+6) found in molybdate or heteropolymolybdate coatings does not exhibit oxidation-reduction potentials comparable to hexavalent chromium (i.e., in acidic media, Mo+6+3e−→Mo+3, potential is +0.43 V; or Mo+6+2e−→Mo+4, potential is +0.65 V). Similarly, hexavalent tungsten (W+6) found in tungstate or heteropolytungstate coatings does not exhibit oxidation-reduction potentials comparable to hexavalent chromium (i.e., in acidic media, W+6+2e−→W+4, potential is −0.12 V). In order to match the oxidation-reduction potential of hexavalent chromium, tetravalent cerium must be present in the coating.
U.S. Pat. Nos. 5,635,084, 5,582,654 and 5,194,138, all to Mansfield et al., describe methods for treating the surface of an aluminum alloy having a relatively high copper content, so as to make the surface resistant to corrosion. The method comprises a) removing substantially all of the copper from the surface of the alloy, b) contacting the surface with a first solution containing cerium, c) electrically charging the surface while contacting with an aqueous molybdate solution, and d) contacting the surface with a second solution containing cerium. However, tetravalent cerium is not disclosed.
European Application No. EP 0 792 922 A1, by The Boeing Company et al., describes chromate-free, corrosion-inhibiting coatings for protection of aluminum and its alloys comprising a) a film-forming organic polymeric and/or sol-gel component, b) an ester of a rare earth metal, i.e., cerium, or a vanadate salt of an alkali or alkaline earth metal, and c) a borate salt of an alkali earth metal. The film-forming organic polymeric or sol-gel component may provide short-term corrosion resistance by functioning as a barrier layer. However, these films usually interfere with substrate oxidation during the conversion coating process and result in thin, incompletely anodized surfaces, resulting in poor mechanical adhesion to the solution-deposited polymer film and to later applied coatings. Polymers added during the coating process also produce smooth coatings with a limited amount of “rough” surface morphology for subsequent paint adhesion resulting in reduced long-term corrosion protection. Neither the importance of tetravalent cerium nor the functional parameters for tetravalent cerium-containing complexes are described.
Similarly, U.S. Pat. No. 5,192,374 to Kindler describes the formation of an aluminum oxide (boehmite) coating on structural aluminum, followed by treatment with a soluble cerium salt and a metal nitrate at 70° C. to 100° C. to form cerium oxides and hydroxides for increased corrosion resistance. The formed oxides and hydroxides are described as filling the pores in the boehmite coating. Also, Stoffer et al. in U.S. Pat. No. 5,932,083 describe the use of a solution containing cerium and an oxidizing agent for treatment of aluminum alloys. The aluminum-containing substrate is electrolyzed in this solution, forming a mixed aluminum oxide/cerium oxide (or hydrated cerium oxide) coating on the aluminum as a barrier film. The formation of tetravalent or hydrated tetravalent cerium oxide is described. However, neither Kindler nor Stoffer et al. teach the use of “valence stabilizers”, which are important for use of tetravalent cerium compounds having aqueous solubilities that are sufficiently high to ensure long-term self healing of the coating. The cerium oxides and hydrated oxides described in these patents function merely as pore-filling barrier layers and not as active self-healing inhibitors within the coating. Moreover, the use of tetravalent cerium oxides and hydroxides as corrosion inhibitors results in lower corrosion performance, as is described herein, due to the fact the electrostatic double layers around such oxide and hydroxide species are much smaller than those exhibited by tetravalent cerium species containing 50% or less oxide or hydroxide as attached ligands.
Similarly, PCT International Publication No. WO 88/06639 by the Commonwealth of Australia and U.S. Pat. No. 6,022,425 to Nelson et al. describe the application of a corrosion-resistant coating for aluminum based on cerium, which cerium is oxidized to the tetravalent oxidation state, resulting in the formation of tetravalent or hydrated cerium oxides. However, these references teach tetravalent cerium compounds having aqueous solubilities that are so low they function as barrier films or sealants, rather than active corrosion inhibitors. Moreover, the use of valence stabilizers for forming complexes with tetravalent cerium is not disclosed.
European Patent Application No. EP 0 902 103 A1 by Nippon Steel Corporation describes the application of a trivalent cerium solution with organic oxoacids to aluminum or galvanized steel. U.S. Pat. No. 6,190,780 B1 to Shoji et al. describes the use of rare earth and/or Group IVA solutions for the treatment of metal surfaces with oxyacids (i.e., molybdates, tungstates, vanadates, or phosphates). Likewise, U.S. Pat. No. 6,200,672 B1 to Tadokoro et al. describes the use of rare earth and/or Group IVA solutions with selected organic molecules for treatment of metal surfaces. U.S. Pat. No. 5,964,928 to Tomlinson describes the use of a Group IVA compound (i.e., zirconium, titanium, or hafnium) in combination with a rare earth element and optionally a fluoride. Also, U.S. Pat. No. 6,503,565 B1 to Hughes et al. describes the use of aqueous acidic, rare earth ion-containing coating solutions for metal surfaces. The coating solutions can include rare earth cations capable of having more than one valence state. However, none of these references teach the presence of a valence stabilized, oxidized rare earth element such as cerium, praseodymuim, or terbium in the formed conversion coating, whose availability to the corroding system is controlled via the solubility of the oxidized rare earth compounds. In order to function as a true replacement for hexavalent chromium, which is itself a highly oxidized species, the rare earth compound must be oxidized in the formed coating.
U.S. Pat. No. 6,206,982 B1 to Hughes et al. describes the use of a four component system to provide corrosion protection of aluminum. One of these components includes a rare earth compound, especially cerium.
The use of colloidal suspensions of tetravalent cerium oxide (CeO2) in anticorrosive coatings is described in U.S. Pat. Nos. 5,733,361 and 5,922,330 to Chane-Ching et al.; PCT International Publication No. WO 96/26255 by Rhone-Poulenc Chimie; and PCT International Publication Nos. WO 01/36331 A1 and WO 01/38225 A1 by Rhodia Terres Rares. The CeO2 exhibits a solubility that is too low for effective release of corrosion-inhibiting tetravalent cerium ions.
An aqueous dispersion of a cerium compound with other rare earths, transition metals, aluminum, gallium, or zirconium is described for anticorrosive agents in PCT International Publication No. WO 01/55029 A1 by Rhodia Terres Rares. Similarly, an aqueous dispersion of cerium oxide in combination with additives such as beta-diketones, alpha-hydroxycarboxylic acids, beta-hydroxycarboxylic acids, or diols is described for anticorrosive agents in U.S. Pat. No. 6,033,677 to Cabane et al. Neither of these references defines the need for cerium to be in the tetravalent oxidation state to achieve anticorrosive effects.
U.S. application Publication Ser. No. 2003/0,024,432 A1 by Chung et al. describes an anti-corrosive surface treatment comprising, inter alia, an organometallic compound that can include cerium (i.e., cerium acetate hydrate, cerium acetylacetonate hydrate, cerium 2-ethylhexanolate, i-propoxycerium, cerium stearate, and cerium nitrate). The disclosed coating is an anti-corrosive sol-gel that produces an adhesive film interface between a metal surface and an organic matrix resin or adhesive. In addition, U.S. application Publication Ser. No. 2003/0,019,391 A1 by Kendig describes a corrosion inhibitor comprising an oxo-anion and a cation that is capable of inhibiting the propagation of pit corrosion on the surface of coated metal substrates. The cation can be a rare earth metal including cerium and praseodymium, inter alia. However, neither Kendig nor Chung et al. describe the need for cerium to be in the tetravalent oxidation state for corrosion inhibition.
Accordingly, a need exists for improved corrosion-protection conversion coatings composed of currently unregulated and/or non-toxic materials which have an effectiveness, ease of application, and performance comparable to coatings formed with hexavalent chromium, and for methods of making and using the same.